A CdS photocatalyst was synthesized successfully at low temperature via a catalyst-free hydrothermal technique which is simple, green and also easily controlled. The synthesized CdS photocatalyst showed hexagonal wurtzite structure with high crystallinity and excellent optical properties. The catalyst was used for degradation of two anionic azo dyes namely reactive red (RR141) and Congo red (CR) azo dyes. The catalyst showed very high efficiency of 99.8% and 99.0% toward photodegradation of RR141 and CR dye, respectively. The photodegradation reaction followed pseudo-first order kinetics. Chemical scavenger studies showed that direct photogenerated hole transfer from CdS to the azo dye was most likely the major pathway for photodegradation of the azo dye. The chemical structure of the CdS photocatalyst remained stable after photodegradation. The CdS photocatalyst retains its original efficiency even after the fifth cycle of reuse indicating the advantages of stability and reusability. The CdS nanostructures will be suitable for removal of highly toxic and hazardous organic materials in environmental protection.